Why are repeated PO4 reducing treatments necessary
The Problem
Frequently hobbyists will send email to various mailing lists requesting how to get rid of green undesirable algae, and want to know the exact approach to take to do so. "What do I need to do to get rid of those slime, hairy, filamentous algae," is the typical question.
This appears to happen here in Europe just as much as it occurs in North America, at least from what I have seen through navigating various mailing lists and newsgroups.
The problem is that over time or, for some specific not yet identified reasons, the water in the tank becomes loaded with organic and other forms of phosphate-based compounds.
When this happens, these undesirable algae grow and spread rapidly covering everything in the aquarium, unless speedy action is taken to lower these phosphate levels.
The real nutrient for these and other forms of algae is ortho-phosphate. This is the mineralized form of any type of phosphate present in the aquarium. Organic phosphates, whether simple or complex ones, break down and mineralize and eventually appear in the aquarium water as ortho-phosphate.
Algae of all sorts thrive on this nutrient and when it is present, rapidly grow and proliferate all over the tank, including on your coralline algae, your coral's skeletons and so on. The result is not a pretty picture indeed.
Course of action
In order to solve this problem the approach needs to be two-fold:
the phosphate already present in the aquarium needs to be removed from the water so algae are starved slowly of nutrients and gradually die off as a result
sources of phosphate need to be identified and eliminated so that the build up does not reoccur. If external sources can be found that artificially add PO4 to the tank, the hobbyist can eliminate these.
There are several methods that are generally suggested for the removal of phosphate from aquariums. The easiest one, but not necessarily the least expensive one is to make water changes (a certain percentage of the tank content) every day. The water that is then added to replace this needs to be free of phosphates of course and needs to have the same general water quality parameters as the water already in the tank (see Thiel, various articles on algae control on http://www.athiel.com and in his books).
Another method suggested by various authors and manufacturers is the use of phosphate-removing compounds. These are usually metal oxides that have been specially treated to absorb ortho-phosphate.
Several brands appear to compete with each other in this area. All would appear to be the same though: aluminium oxide treated in special manners, then agglomerated and sold in powder, granules, chunks, balls and so on (even impregnated pads are available).
Either of these methods will eventually bring the phosphate levels down and prevent algae growth after having allowed you to rid the aquarium of the algae that were already there.
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